This invention relates generally to a vacuum fluorescence display (VFD) for displaying visual information, and more particularly to an improved VFD assembly and method of manufacture.
VFD displays using a glass vacuum envelope are known. Since a VFD is essentially a triode tube having a hot filament, an electron smoothing grid and phosphorous anodes, the same techniques used for making electronic vacuum tubes are commonly applied to the manufacture of VFDs.
Standard VFDs are thus generally constructed entirely of glass as are electron vacuum tubes. However, use of glass in VFD construction introduces several difficulties. Glass is heavy relative to other possible materials. Additionally, to provide sufficient VFD durability, the thickness of the walls of a glass VFD must be substantial. A light shield is required to prevent light from penetrating the VFD in undesired directions such as the sides and back of the VFD. Glass can accumulate static charges which can interfere with operation of low-voltage VFDs. Finally, glass VFDs are sealed with a glass frit that tends to out-gas substantially, leading to degradation of the internal vacuum inside the VFD.
The use of glass in standard VFDs also includes the base on which the phosphor screen is mounted. Glass is a non-ideal material for this application because of relatively high costsxe2x80x94glass formations require carbon jigs that have a short life at the glass melting temperature even in an inert atmosphere. Sintering temperatures in a glass VFD are approximately 500xc2x0 C. At this relatively low temperature, metallic patterns printed on the glass surface are attached mainly by frit fixing, a process that does not give ideal adhesion and which leads to lower conductivity and potential oxidation of the electronic contacts. Additionally, only one layer of circuitry can be printed on a glass surface.
Electronic contacts connecting internal circuitry to outside the VFD are also a serious weakness of standard VFD design. The weak point of the vacuum package is where the metal leads pass through the glass frit from the inside of the vacuum package to the outside. The getter in the package is fired using RF energy, which heats up the getter to the point where it fires (the getter is like molecular fly paper and absorbs gasses that outgas from the surfaces inside the vacuum envelope and that continually reduce the vacuum after the package is sealed). In a small VFD package, the lead frame is positioned too close to the getter and also heats up during getter firing. This can cause a thermal fracture to occur in the frit around the lead frame, thus causing the package to lose vacuum and fail.
In standard VFDs constructed of glass, the getter is generally placed beside the phosphor screen. This placement requires additional VFD footprint that is not usable for displaying visual information and may lead to problems of screen contamination by the getter material during getter firing. Standard getters are of the evaporation typexe2x80x94during firing of the VFD envelope, the getter sputters barium metal (typically) throughout the interior of the vacuum package. This can lead to further contamination. Technology that allows the getter to be placed after firing would solve this problem.
Accordingly, it is an object of the present invention to provide an improved vacuum fluorescence display (VFD) constructed substantially out of an alternate material than glass. The display includes metal sidewalls and fluorescent material carried on one or more silicon wafers. A ceramic, layerable insulating material is employed as the substrate on which the wafer or wafers are mounted. Standard VFDs use glass for both the sidewalls and the substrate base for the silicon wafer or wafers. In another embodiment of the current invention, a chemical getter is incorporated into a recess formed in the ceramic substrate. The getter is positioned underneath the backside of the phosphor screen so as to significantly reduce contamination of the screen by material sputtered off of the surface of the getter.
More specifically, the present invention provides an improved vacuum fluorescence display formed of a ceramic substrate, a metal collar sealed to one surface of the ceramic substrate, and a viewing window disposed approximately parallel to the ceramic substrate and sealed to the metal collar to form a sealed, evacuated interior volume. Inside this sealed interior volume, there is a phosphor screen mounted on the ceramic substrate. The phosphor screen is formed of phosphor pixels disposed in a grid pattern on one or more semiconductor chips which contain integrated circuits that form a matrix of anodes. Via the internal circuitry of the semiconductor chips, these anodes can be individually energized to activate the phosphor screen in a desired pattern. The semiconductor chips are powered and controlled by a plurality of electronic connections. The VFD also includes a getter formed of a chemically reactive material that is mounted inside of the sealed, evacuated interior volume and beneath the phosphor screen. A number of filament cathodes inside the sealed interior volume between the viewing window and the phosphor screen provide a source of free electrons that are accelerated toward the phosphor screen by a mesh biasing grid positioned between the filament cathodes and the phosphor screen.